The present invention relates to the field of stencil printing in general and more particularly to the duplicator stencil process.
The duplicator stencil used in this process is an ink impervious layer in which holes are made through which ink can be fed onto paper. Whilst this process has been used for many decades, the means of making these holes in the stencil have changed considerably. The earliest method was to use a writing stylus and with the invention of type-writers, the impact of the type was used to make the hole. Subsequently, the impervious layer was electrically eroded with a spark and alternatively pierced by reflex infra-red radiation. For the latter two methods, the information was supplied in the form of written matter on paper.
With the advent of the computer, material was then available in a digital form, and stencil duplicators were developed into stencil printers. This was done utilizing the thermal stencil design previously developed for reflex infra-red radiation sensitive material. Such stencils consist of a xe2x80x9cYoshinoxe2x80x9d type tissue with a thin pre-stretched film which may be polyvinylidene chloride, such as the commercially available xe2x80x98Saranxe2x80x9d type, manufactured by Dow Plastics of Midland, USA, bonded to the surface of the tissue. The stencil is imaged by a thermal contact head where each thermal point is digitally controlled to heat up to correspond to an image point. A suitable stencil printer is described in U.S. Pat. No. 4,628,813 to Hageyama. The heated pre-stretched film retracts, leaving a hole through which ink can pass. This process differs in quality from the previous reflex method although the stencil material is still the same. The previous method was restricted in quality because it relied on the difference in heat between background and image on a piece of printed paper that was held in contact with the film during exposure. The printed area needed to be heat absorptive and only carbon black based inks worked well.
An alternative method of imaging the stencils is by using an infra red laser. Infra red laser methods differ from the previous method in that the head does not comes into contact with the stencil. Thus, there is no wear from contact and the head cannot be contaminated by film as may happen with conventional thermal heads. It is easier to place small image spots closer together to obtain high resolution imaging.
Reference is made to FIG. 1, which illustrates the component layers of a prior art thermal stencil sheet, generally designated 10, which may be perforated using laser beams.
Stencil sheet 10 generally comprises several layers including a base layer 12, an adhesive layer 14, a thermal film 16 and a top coated surface 18. Base layer 12 is generally a porous fibrous layer such as non-woven paper tissue, for support of the perforated film. Adhesive layer 14 adheres the thermal film 16 to the base 12. Thermal film 16 comprises a suitable polyester film material, such as polyethylene terephthalate or polyvinylidene chloride. The coated surface 18 is generally used to protect the thermal film 16 from fusing and being damaged due to friction and may comprise a silicon fluorine mold lubricant, for example.
When a laser beam is irradiated to a thermal stencil sheet made of a heat-sensitive plastic film having a relatively high transparency, most of the laser beam merely passes through the heat-sensitive plastic film. There are several methods for applying the laser energy to the film, all of which involve the conversion of the light energy to heat. Therefore, in order to apply a heating effect to such a thermal stencil sheet by a laser beam sufficient to cause a perforation thereof, a laser beam is required to have an extremely high energy density. The cutting of the film is influenced by the structure of the base material. Being a non-uniform substance, it has a different effect on different areas of the film, as much as heat diffusion, mechanical stress and adhesion are concerned. As a result, the film areas being cut due to the laser heating have poor definition. Small details can not be reproduced at all.
The non uniform structure of the base is required for the ink transfer in the printing stage. It is therefore an essential attribute of the base material and can not be modified.
FIG. 2 illustrates in a magnified cross section the condition of perforation formed in the thermal film 16 by a conventional minute thermal element (indicated by arrows 17) pressed against the heat-sensitive film from its front side. A part of the film fused by the thermal head sticks to the head resulting in a cone shape (19) having diameter increasing toward the front side.
GB Patent No. 1,357,988 assigned to Riso Kagaku Corporation of Japan, describes the use a black stencil sheet having fine particles of a light absorbing heat generating substance, such as carbon, distributed in a heat-sensitive plastic film. In order to produce a fine stencil print by using such a stencil sheet, the light absorbing heat generating substance needs to be distributed at high density and uniformity in the heat-sensitive plastic film. Since it is not possible to strongly bind the particles to the film by chemical means, the fine solid particles are held in the plastic layer by mechanical means, which is not effective. Thus, the stencil sheet is not suitable for use with a laser beam having low energy density.
U.S. Pat. No. 5,483,883 to Hayama describes a method for imaging a stencil using a relatively low energy laser and light absorbing ink. Reference is now made to FIG. 3 which illustrates a stencil sheet 50 mounted onto a printing drum and held thereto by a layer of black ink. The stencil sheet 50 has a heat sensitive plastic film 52 and a net material 54 laid one over the other and bound together. When a laser beam 51 is irradiated to the heat-sensitive plastic film 52 of the stencil sheet backed by the black ink layer 56 attached to the rear surface thereof, most of the laser beam passes through the heat-sensitive plastic film 52 so as to reach the black ink layer 56 where it is absorbed. The temperature of the ink at the irradiated portion rapidly increases, causing heat-sensitive plastic film to melt thereby forming a hole, starting from the rear surface thereof.
FIG. 4 illustrates the type of perforation formed in a heat-sensitive plastic film such 52 by a laser beam irradiated from its front side. The perforation formed by the heat-sensitive plastic film is melted by the heat generated in the ink layer 56 existing on the rear side of the plastic film. In this case, the bore of the perforation has a cone shape having a diameter increasing (from d1 to d2) towards the rear side.
Unfortunately, since specifically infra red absorbing materials are not available, the stencil requires high infra red radiation to image.
The applicants have discovered that by applying a very thin resin based film containing infra red absorbing material, excellent accurate imaging can be achieved using low cost lasers, such as laser diodes.
An object of the present invention is to provide an improved stencil sheet for use with a thermal head which overcomes the limitations and disadvantages of prior art stencil sheets.
There is thus provided, in accordance with an embodiment of the invention, a thermal stencil sheet which includes a thermal film attached by means of an adhesive to a base layer and a coated surface applied to the thermal film. The coated surface contains a resin based film having infra red (IR) absorbing material dispersed therein.
Furthermore, in accordance with an embodiment of the invention, on irradiation of the stencil sheet with infra red laser rays, a perforation having a bore of uniform cross-section is formed extending through the film.
Additionally, there is provided, in accordance with an embodiment of the invention, a method of preparing a thermal stencil sheet. The method includes the steps of:
a. attaching a thermal film to a base layer; and
b. applying a coated surface to the thermal film, the coated surface containing a resin based film having infra red (IR) absorbing material dispersed therein.
Additionally, in accordance with an embodiment of the invention, the IR absorbing material includes any of the following group: carbon blacks, iron oxide and infra red absorbing dyes.
Furthermore, in accordance with an embodiment of the invention, the resin based film includes any of the following group: polyurethane, nitro cellulose and cellulose acetate butyrate.
Additionally, there is provided, in accordance with an embodiment of the invention, a stencil printing system utilizing the thermal stencil sheet of the invention. The system includes laser source means for supplying an infra red (IR) laser beam, control means for controlling the laser source means and a stencil sheet. On exposure to the IR laser beam, the stencil sheet is perforated according to instructions sent by the control means to obtain a desired pattern.
Additionally, there is also provided, in accordance with an embodiment of the invention, a thermal stencil sheet which includes a base layer, a radiation absorbing layer containing a resin based film having infra red (IR) absorbing material dispersed therein and a thermal film overlying the radiation absorbing layer.
Furthermore, in accordance with an embodiment of the invention, the radiation absorbing layer is attached by means of an adhesive to the base layer.
Furthermore, in accordance with an embodiment of the invention, the radiation absorbing layer further includes an adhesive mixed therein for adhering the radiation absorbing layer to the base layer.
Additionally, in accordance with an embodiment of the invention, the thermal stencil sheet further includes a topmost layer overlying the radiation absorbing layer and an intermediate layer underlying the radiation absorbing layer, wherein the intermediate layer is attached by means of an adhesive to a the base layer. The radiation absorbing layer is attached by means of an adhesive to the intermediate layer.
Alternatively, the radiation absorbing layer further includes an adhesive mixed therein for adhering the radiation absorbing layer to the intermediate layer. The adhesive is a ketone solvable glue.
Furthermore, in accordance with an embodiment of the invention, the stencil base includes porous fibrous material which is saturatable with liquid. The liquid includes any of a group including water, petroleum solvent, Toluene and printing ink.
Finally, there is provided a stencil printing system utilizing the thermal stencil sheet of the invention. The stencil printing system includes laser source means for supplying an infra red (IR) laser beam, control means for controlling the laser source means; and a stencil sheet described.